It seems that the old tram depot has a retirement date of 1950, and the new tram depot has an introduction date of 1957, leaving a 7 year gap without tram depots. I have fixed it in the sources of Pak128.Britain-Ex by setting the retirement date of the old tram depot to July 1957, and likewise the introduction date of the new tram depot, and suggest that a like fix be applied to the standard version.
In Standard pak128.Britain, there is no problem - the old retires in 1950 and the new is introduced in 1950 (check the SVN...). Not sure where the 1957 in experimental came from then?
Hmm, odd. Perhaps I altered it thinking that 1957 is a better year to transition to the new depot than 1950 - sorry for the false alarm, in that case.